SE523487C2 - Procedure for controlling a rectifier - Google Patents

Abstract

The invention relates to a method for controlling a VSC-converter provided with a resonant circuit (16). In connection with the effectuation of an intended commutation process ordered by the modulator (30), the control device (24) is made to send control signals to the current valves (2, 3) and auxiliary valve (18) that are taking part in the commutation process, for turning on or turning off thereof, at instants (t1) determined on the basis of a desired commutation instant (ttr) given by the modulator (30) and a calculation algorithm, which is based upon values of the phase current and the intermediate link voltage and knowledge about the influence the components included in the converter have on the intended commutation process, said calculation algorithm being elaborated in consideration of the condition that the desired commutation instant (t*tr) is to coincide with an equivalent transition instant (ttr) for the phase voltage, which equivalent transition instant (ttr) is estimaged with the aid of knowledge about the influence the components included in the convernter have on the transition of the phase voltage (uph(t)) during the intended commutation process.

Description

1: 1 »- 10 15 20 25 30 35 o o -. v, 523 487 l 'I 2 - »» | v,,,., electrical network or a network without any own generation (a dead AC network). Additional benefits are also available.

A VSC converter can be included in a plant for the transmission of electric power via a direct voltage network for high-voltage direct current (HVDC), for example to transmit the electric power from the direct voltage network to an alternating voltage network. In this case, the converter has its DC side connected to the DC network and its AC side connected to the AC network. However, the converter can also be directly connected to a load such as a high-voltage generator or motor, the converter having either its DC voltage side or its AC voltage side connected to the generator / motor. The invention is not limited to these applications, but the converter may just as well be intended for conversion in an SVC (Static Var Compensator) or a back-to-back station.

The voltages on the DC side of the converter are advantageously high, 10-400 kV, preferably 130-400 kV. The VSC converter can also be included in other types of FACTS (FACTS = Flexible Alternating Current Transmission) devices than those listed above.

In order to limit the extinguishing losses in the extinguishing semiconductor elements of the converter current valves, i.e. the losses in the extinguishing semiconductor elements when they are extinguished, it is previously known to arrange capacitive means in the form of so-called snub capacitors connected in parallel across respective extinguishing semiconductor elements. It is also known to provide the converter with a so-called resonant circuit for recharging the said snub capacitors in connection with commutation of the phase current. This also makes it possible to limit the ignition losses in the extinguishing semiconductor elements of the current valves, ie the losses in the extinguished semiconductor elements when they are ignited.

A number of different types of substantially lossless commutation circuits based on inductances and capacitances have been developed and put into use to reduce the losses of VSC converters in connection with commutations. These types of converters are called in English "soft switched converter". As examples of such: rule 10 15 20 25 30 35 n v o ~ -n 523 487 ä e o oou now converters can be mentioned ARCP converters (Auxiliary Resonant Commutated Pole Converter). These converters comprise a resonant circuit which is arranged to cause recharging of the current capacitors of the current valves in connection with commutation of the phase current from a rectifier means of a current valve to a quenchable semiconductor element of another current valve so that said semiconductor element can be ignited at low voltage. , thereby limiting the ignition losses in the semiconductor element of the current valve. The resonant circuit is also used when commutating the phase current from a switchable semiconductor element of one current valve to a rectifier means of another current valve, ie when switching off a semiconductor element of the first current valve, when the phase current is so low that the switching voltage voltage would otherwise become unreasonably long.

The solution according to the invention is generally applicable to converters of the "soft switched converter" type, such as, for example, to the above-mentioned types of converters provided with a resonant circuit.

As further examples of converter types to which the inventive solution is applicable can be mentioned three-level ARCP converters of the type described, for example, in "Three Level Auxiliary Resonant Pole Commutated Pole Inverter for High Power Applications", Cho JG, Baek JW, Yoo DW, Won CY, IEEE, 1996, and quasi-resonant PWM converters of the type described, for example, in U.S. Pat. No. 5,574,2418.

In a converter which does not have a resonant circuit or any snubber capacitors, the commutation of the phase current takes place from, for example, a switchable semiconductor element of a first current valve to a rectifier means of a second current valve in principle at the same moment as the semiconductor element of the first current valve is switched off. switches from one pole voltage to the other in principle at the same moment as the first current valve goes out. There is normally a smaller time delay between the time when a switch-off signal to a current valve is emitted from the converter control device and the time when the switch-off of the current valve, ie the switch-off of Jana: the switch-off semiconductor element of the current valve, really enforced.

If this time delay is disregarded, the control device can in this case consequently be arranged to emit a switch-off signal to the outgoing current valve at the desired commutation time given by the PWM modulator. With a corresponding commutation of a "soft switched converter" type converter, the commutation process becomes relatively slow, whereby the change of the phase voltage takes place relatively slowly. In addition, the change of the phase voltage during the commutation process does not normally take place linearly in time with a converter of the type mentioned. This can result in the output voltage ordered by the modulator not being realized correctly by the inverter, which in turn can lead to the following problems: - Instability in intermediate link voltage control and phase current control. (The converter is generally included in a control circuit for controlling these quantities.) - Increased harmonic content in the phase voltage and thus indirectly in the phase current.

- Unwanted undertones in the phase voltage, which can result in unwanted phase currents and in saturation of transformers connected to the inverter.

- Poor utilization of the converter's ability to deliver phase voltage.

OBJECT OF THE INVENTION An object of the present invention is to provide an improved method for controlling a VSC converter provided with a resonant circuit, which method makes it possible to reduce the above-mentioned problems.

SUMMARY OF THE INVENTION ||| »| 10 15 20 25 30 35 ø o v n oo 525 487 I l 5 v - en u; According to the invention, said object is achieved by means of a method according to claim 1.

The solution according to the invention is based on the fact that this type of resonant circuit in question is built up of substantially linear elements, such as inductors and capacitors, which makes it possible to use simple mathematical relationships to determine suitable times for transmitting control signals to the current valves and the auxiliary valve. This makes it possible to calculate before the initiation of a commutation when the various control signals are to be sent to current valves and auxiliary valves participating in the commutation process so that the time of the phase voltage transition from one to the other pole voltage coincides with the desired module. tower ordered the commutation time.

According to a preferred embodiment of the invention, the times for emitting control signals to the current valves and the auxiliary valve are calculated on the basis of the value of the voltage across the intermediate link, the value of the phase current, the value of the snubber capacitance and the value of the inductance of the resonant circuit. The times for outputting control signals from the control device are thus determined on the basis of parameters which are constant or at least substantially constant during the respective commutation processes, such as the snubber capacitance and inductance of the resonant circuit, or which vary relatively slowly during the respective commutation course and phase current. , why a relatively good accuracy can be achieved in the calculation of said times.

According to a further preferred embodiment of the invention, the variation of the phase current during the commutation process is taken into account in the calculation of said times. This results in increased precision in the steering. The varying value of the phase current during the commutation process is preferably estimated with the aid of a linear model of the load connected to the phase socket. 10 15 20 25 30 35 ø v u u co -523 487 'é' n - u n n n «: u u; Further preferred embodiments of the method according to the invention appear from the dependent claims and the following description.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described in more detail below by means of exemplary embodiments, with reference to the accompanying drawing.

It is shown in: Fig. 1 Fig. 2 Fig. 3-5 Fig. E Fig. 7 Fig. Fig. 9 a simplified circuit diagram illustrating a converter according to a first embodiment, a simplified circuit diagram illustrating a converter according to an alternative embodiment, current and voltage curves during different commutation processes, a simplified block diagram illustrating a control system for carrying out the method according to the invention, a curve showing the change of the phase voltage during an ideal commutation process, a first curve showing the change of the phase voltage during a commutation process of a resonant circuit equipped with a resonant circuit. and a second curve showing the change of the phase voltage during a commutation process of a converter equipped with a resonant circuit. iriz: 10 15 20 25 30 35 u o n Q co v 523 487 '' 7 n e av: on DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS l Fig. 1 illustrates a VSC converter. Fig. 1 shows only the part of the converter which is connected to a phase of an alternating voltage phase line, the number of phases normally being three, but it is also possible that this constitutes the entire converter when it is connected to a single-phase AC mains. The shown part of the converter constitutes a so-called phase leg and a converter adapted for, for example, a three-phase AC voltage network comprises three phase legs of the type shown.

VSC converters are known in several embodiments. In all embodiments, a VSC converter comprises a number of so-called current valves, each of which comprises a quenchable semiconductor element, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), and an antiparallel thus connected rectifier means in the form of a diode, a so-called freewheel diode.

Each high-voltage semiconductor element is normally composed of a number of series-connected, simultaneously controlled extinguished semiconductor components, such as a number of individual IGBTs or GTOs. Namely, in high voltage applications, a relatively large number of such semiconductor components is required to maintain the voltage that each current valve must maintain in the blocked state. Correspondingly, each rectifier means is made up of a number of rectifier-connected rectifier components. The extinguishable semiconductor components and the rectifier components of the current valve are arranged in a plurality of series-connected circuits, which circuits each comprise, among other things, an extinguishable semiconductor component and a rectifier component connected thereto in parallel.

The phase leg of the converter 1 illustrated in Fig. 1 has two current valves 2, 3 connected in series between the two poles 4, 5 of a direct voltage side of the converter. An intermediate link 6 comprising at least one so-called intermediate link capacitor is arranged between the two poles 4, 5. In the a-iaa 10 15 20 25 30 35 a ø ø p now 1523 487 8 n now illustrated by the converter the intermediate element 6 comprises two series-connected intermediate capacitors 7, 8. A center point 9 between these capacitors 7, 8 is here, as usual, connected to earth, so that in this way the potentials + U ,, / 2 and -Ud / Z provided at the respective pole, where U, is the voltage between the two poles 4, 5. The ground point 9 can, however, be omitted, for example in SVC applications.

A center point 10 of the series connection between the two current valves 2 and 3, which constitutes the phase socket of the converter, is connected to an alternating voltage phase line 11. In this way said series connection is divided into two equal parts with a current valve 2 and 3 of each such part. In the embodiment with three phase legs, the converter thus comprises three phase outlets, which are each connected to an alternating voltage phase line of a three-phase alternating voltage network. As a rule, the phase sockets are connected to the AC mains via electrical equipment in the form of switches, transformers, etc.

In the embodiment shown, the respective current valves 2, 3 according to what has been stated above comprise a plurality of series-connected circuits 12, which circuits each comprise an extinguishable semiconductor component 13, such as an IGBT, an IGCT, a MOSFET, a JFET, a MCT or a GTO, and an anti-parallel rectifier component 14 in the form of a diode, a so-called free-wheel diode. In the embodiment shown in Fig. 1, each flow valve 2, 3 comprises two series-connected circuits 12 of the type described above, but the series-connected circuits 12 may be more or less in number. Depending on, among other things, the voltage for which the converter is designed, the number of said series-connected circuits 12 in the respective current valve 2, 3 can range from two up to several hundred.

Each of the series-connected circuits 12 of the respective current valve 2, 3 is provided with a capacitor 15, herein referred to as the snubber capacitor, connected in parallel with the extinguishing semiconductor component 13 included in the circuit. The capacitance of the respective snubber capacitor 15 must be so high that a god> |||| 10 15 20 25 30 35 | Voltage distribution between the extinguishable semiconductor components 13 included in the respective current valve is made possible when the extinguishing semiconductor components of a current valve are extinguished. The choice of capacitance of the snubber capacitors 15 is adapted from case to case and depends, among other things, on the current handling capability of the extinguished semiconductor components 13 and the rectifier components 14. The snubber capacitors 15 help to limit the extinction losses of the current valves, ie the losses in the extinguishable semiconductors. goes out.

When the semiconductor components 13 of a current valve are switched off, the snubber capacitors 15 which are connected across these semiconductor components 13 will be charged. If the snubber capacitors 15 retain this charge when the semiconductor components 13 are then ignited, ignition losses occur at the semiconductor components 13. The relatively high-capacitive snubber capacitors 15 which come into question in this case give rise to very high ignition losses, which ignition losses makes it impossible to use high switching frequencies. In order to eliminate or at least reduce these ignition losses, and enable the use of high switching frequencies, the snub capacitors 15 are included in a resonant circuit 16. This makes it possible to discharge a snub capacitor 15 of a current valve when the semiconductor components 13 of the current valve shall be ignited so that the voltage across the respective semiconductor component 13 is equal to or close to zero when ignited, thereby limiting the ignition losses.

It is also possible to include a capacitor arranged between the phase terminal 10 and the center point 9 of the direct voltage intermediate joint in the resonant circuit 16.

The resonant circuit 16 here is of the so-called quasi-resonant type, which means that the resonance is only initiated in connection with the current being commutated between two current valves, i.e. when the voltage on the phase socket of the converter is to be switched. Resonance circuits of this type are known in a number of embodiments. For example, 523 487 523 487 1D describes the use of a quasi-resonant so-called ARCP circuit of the embodiment illustrated in Fig. 1 in US 5047913. Although the method according to the invention below will be exemplified in the application of a two-level ARCP -converters, it is emphasized that the method is nevertheless applicable for controlling any other soft-switched converter of the "soft switched converter" type, such as, for example, converters of previously exemplified embodiments.

In this type of converter in question, a resonant circuit is used comprising at least one inductor and an auxiliary valve provided with extinguishing semiconductor components for recharging the snub capacitors of the current valves in connection with commutation of the phase current.

The term auxiliary valve in this description and subsequent patent claims refers to a current valve included in the converter's resonant circuit 16.

In the embodiment shown in Fig. 1, the resonant circuit 16 comprises a series connection of an inductor 17 and an auxiliary valve 18 arranged between the phase socket 10 and the center point 9 of said series connection of intermediate conductor capacitors 7. The auxiliary valve 18 here comprises a set of two series-connected auxiliary valves. circuits 19, each of which comprises a quenchable semiconductor component 20, such as an IGBT, an IgCT, a MOSFET, a JFET, an MCT or a GTO, and a rectifier component 21 connected thereto in the form of a diode. The quenchable semiconductor components 20 of the two auxiliary valve circuits 19 are arranged in opposite polarity with respect to each other. This auxiliary valve 18 constitutes a bidirectional valve which can be caused to lead in one or the other direction.

The auxiliary valve 18 may also include several series-connected sets of auxiliary valve circuits if appropriate, as illustrated in Fig. 2. In the embodiment illustrated in Fig. 2, the resonant circuit 18 comprises an auxiliary valve 18 comprising a plurality of series 1523 487 .s = - '1 .vu of 1'1 connected sets 22 of auxiliary valve circuits, each set comprising two series-connected auxiliary valve circuits 19 of the type described above. Fig. 2 shows only two series-connected sets 22 of auxiliary valve circuits of the auxiliary valve 18, but the number of such sets can be considerably larger than that. The number of sets of auxiliary valve circuits of the auxiliary valve 18 can be optimized independently of the number of series-connected circuits 12 of the current valves 2, 3, and depends, inter alia, on what voltage the auxiliary valve should be able to maintain in the blocked state and the properties of the individual semiconductor components 20 used.

In general, it can be stated that the auxiliary valve 18 in the blocked state only needs to keep half the pole voltage, i.e. Ud / 2, in contrast to the current valves 2, 3, each of which must be dimensioned in order to be able to keep the entire pole voltage U, in the blocked state.

Each set 22 of auxiliary valve circuits 19 of the auxiliary valve 18 is suitably, as illustrated in Fig. 2, provided with its own control unit 23 which is arranged to control the ignition and extinguishing of the extinguishable semiconductor components 20 included in the set, all control units 23 of the auxiliary valve being connected to a common control device 24 which is arranged to send control signals to all these control units 23. This ensures a simultaneous control of all the auxiliary valve circuits 19.

It is further preferred that each of the extinguishable semiconductor components 13 included in the converter current valves 2, 3, as illustrated in Fig. 2, is provided with its own control unit 25 which is arranged to control the switching on and off of the semiconductor component 13, all control units 25 of the current valves are connected to a common control device 24, which is arranged to send control signals to all control units 25 included in a current valve 2, 3. This ensures a simultaneous control of all semiconductor components 13 of a current valve.

The control units 23 of the auxiliary valve and the control units of the current valves 1:11) are here connected to one and the same control device 24, which is preferred.

There are three basic processes for commutating the phase current of a converter of the type illustrated in Figs. 1 and 2, which basic processes will be briefly described in the following.

The flow valve which is initially energized, i.e. when the commutation process is initiated, is referred to in this description and in the following claims as "the first flow valve" and the flow valve to be energized by the commutation is referred to as "the second flow valve". It will be appreciated that which of the two flow valves 2, 3 illustrated in Figs. 1 and 2 which at each specific commutation occasion constitute "the first" and "the second" flow valve, respectively, varies from case to case.

In this description and in the following claims, the expression "a commutation not assisted by the resonant circuit" means that the series connection of the auxiliary valve 18 and inductor 17 included in the resonant circuit does not participate in the commutation process. Of course, however, the capacitive means of the resonant circuit, i.e. the snubber capacitors 15, participate in this commutation process. Correspondingly, the expression "a commutation assisted by the resonant circuit" means that the series connection of auxiliary valve 18 and inductor 17 included in the resonant circuit participates in the commutation process.

A first commutation process involves commutating the phase current from a quenchable semiconductor element of a energized first current valve 2, 3 to a rectifier means of a second current valve 3, 2 without the assistance of the resonant circuit 16. The commutation process is initiated by the quenchable semiconductor element of the the first current valve is switched off, whereby the phase current ip, causes a charging of the capacitive means of the first current valve, i.e. its snubber capacitors 15, and a discharge of the capacitive means of the second current valve, i.e. its snubber capacitors 15. The phase potential then becomes v ~ | vn 10 15 20 25 30 35 523 487 13 .-- »- to swing from one pole to the other pole. Fig. 3 illustrates the change of the current i, .g ,,, and the voltage uígb, of the extinguishing semiconductor element of the first current valve during the commutation process. The extinguishing semiconductor element of the first current valve is extinguished at time fo, whereby the current through the semiconductor element ideally goes directly down to zero.

In reality, the semiconductor element will have a certain conduction current. The phase current iph will then cause a charge of the capacitive means of the first current valve, the voltage across it and thus over the extinguishable semiconductor element increasing substantially linearly from zero to a value Ud corresponding to the voltage between poles 4, 5. It will be appreciated that the extinction of the semiconductor element of the the first flow valve can be made substantially without any power losses.

The duration T, of this commutation process is ideally given by the formula where Ud is the voltage across the series connection 6 of intermediate capacitors, ip. is the phase current and C, is the snubber capacitance, i.e. the sum of total, series-connected snubber capacitance for one valve 2, total, series-connected snubber capacitance for the other valve 3 and, where applicable, the capacitance of the capacitor arranged between the phase outlet 10 and the intermediate point 9.

A second commutation process involves commutating the phase current from a quenchable semiconductor element of a energized first current valve 2, 3 to a rectifier means of a second current valve 3, 2 with the assistance of the resonant circuit 16. This commutation process is used at low phase currents to accelerate the commutation. The commutation process is initiated by the extinguishing semiconductor component (the embodiment according to Fig. 1), arsa: 10 15 20 25 30 s23 487 1'4 now or, where applicable, the extinguishing semiconductor components (the embodiment according to Fig. 2), which are energized (-a) the auxiliary valve lights up. At the same time as the semiconductor component (s) of the auxiliary valve lights up, the semiconductor element of the first current valve goes out. In the event that so-called boost current is used in connection with the commutation, the semiconductor element of the first current valve is switched off slightly after the semiconductor component (s) of the auxiliary valve are switched on. In the following, it is assumed that a method without the use of boost current is applied. A resonant period now takes place during which the resonant circuit provides a current to the phase socket 10 which contributes to charging the first current valve snub capacitors 15 and discharging the second current valve snub capacitors 15. After the voltage across the second current valve has dropped to zero or to a value close to zero, the extinguished semiconductor element of the second current valve lights up. After the current in the resonant circuit has dropped to zero or to a value close to zero, the quenchable semiconductor component (s) 20 which was initially turned on by the auxiliary valve 18 are extinguished.

For a case with a balanced intermediate, ie when the voltages ud, and u "across the intermediate capacitors are equal (ud, = u ,,, = Ud / 2), the duration T ,, of this commutation process is given by the formula: zz - 'Tu = i rr-Zarctan ---- ° Ilphl wo Ulf. .. 1 l above formulas ar wo = L. and Zo = Cm, where Lm res .t S is the value of the inductance of the resonant circuit and Cs as previously mentioned is the snubber capacitance.

The above formula for T ,, is suitably adjusted for a case of unbalanced intermediate, i.e. when the voltages ud, and ud: across the intermediate capacitors are different sizes. : nano 10 15 20 25 30 35 n n p »nu 523 487 15 Fig. 4 illustrates the change of the current im through the resonant circuit and the phase voltage out, during the commutation process described above. The extinguishing semiconductor component (s) of the auxiliary valve 18 lights up at time 1,. In the case illustrated in Fig. 4, no boost current is used, so the extinguishing semiconductor element of the first current valve is extinguished at the same time 1.

A third commutation process involves commutating the phase current from a rectifier means of a energized first current valve 2, 3 to a quenchable semiconductor element of a second current valve 3, 2 with the assistance of the resonant circuit 16. The commutation process is initiated by the quenchable semiconductor component (the embodiment according to Fig. 1), or, where applicable, the extinguished semiconductor components (the embodiment according to Fig. 2), which are energized (-a) of the auxiliary valve, are ignited, whereby a so-called ramp-up period begins. During the ramp-up period, the current in the resonant circuit increases, under the influence of the voltage uu. ud, across either of the intermediate capacitors 7, 8, from zero to a value corresponding to the phase current idd. Ideally, the voltages out, and out, across the intermediate capacitors are equal, but in practice they may differ slightly from each other. The duration Td, of the ramp-up period is ideally given by the formula where LM is the value of the inductance of the resonant circuit and ud, the voltage ud, between a first 4 of the poles and the center point 9 of the intermediate capacitors, corresponds to the voltage across the intermediate capacitor 7, , from a current valve 2 arranged between said first pole 4 and the phase terminal 10 to a current valve 3 arranged between the second pole 5 and the phase terminal 10, while ud, the voltage ud, corresponds to the center point 9 of the intermediate capacitors and the second: Juan 10 15 20 25 30 n When the commutation of the phase current ip, from a current valve 3 arranged between the second pole 5 and the phase terminal 10 to a current valve 2 arranged between the first pole 4 and the phase terminal 10. I ideally, Lm corresponds to the value of the inductance of the inductor of the resonant circuit, but in practice the inductance LM of the resonant circuit is also affected by inductance. r of other components of the resonant circuit.

When the current through the resonant circuit reaches a value corresponding to the phase current ip, a resonant period occurs during which the snub capacitors 15 of the first current valve are charged and the snub capacitors 15 of the second current valve are discharged. When the voltage across the second current valve has dropped to zero or to a value close to zero, the extinguishing semiconductor element of the second current valve lights up. The duration TM of the resonance period is ideally given by the formula __ 71 'TFES _ i? mo däfcoo = IC.

S L 76A 'After the resonant period, a so-called ramp-down period occurs during which the current in the resonant circuit decreases to zero from a value corresponding to the phase current iph. The duration TW, of the ramp-down period is ideally given by the formula where ud, the voltage ud, corresponds to the midpoint 9 of the intermediate capacitors and the second pole 5, i.e. the voltage across the intermediate capacitor 8, when commutating the phase current ip, from a current valve 2 arranged between the first pole 4 and Io | »; 10 15 20 25 30 35 »~ o Q nu u 523 487 0- nu 17 the phase terminal 10 of a current valve 3 arranged between the second pole 5 and the phase terminal 10, while ud, corresponds to the voltage ud, between the first pole 4 and the center of the intermediate capacitors. point 9, d vs the voltage across the intermediate capacitor 7, when commutating the phase current 1 ,, from a current valve 3 arranged between the second pole 5 and the phase terminal 10 to a current valve 2 arranged between the first pole 4 and the phase terminal 10.

When the ramp-down lead is over and the current has dropped to zero in the resonant circuit, the quenchable semiconductor component (s) which was initially ignited at the auxiliary valve 18 is extinguished. Fig. 5 illustrates the change of the current im, by the resonant circuit and the phase voltage u ,,,, during the commutation process described above.

The control device 24 is supplied with signals representing the desired commutation times rf, from a modulator 30 schematically illustrated in Fig. 6, which is preferably a PWM modulator (PWM = Pulse Width Modulation). In an ideal case, an ordered commutation takes place directly in a single step at the commutation time z 1 'indicated by the modulator, as illustrated in Fig. 7, i.e. the phase voltage passes in a single step from one pole voltage to the other. In a converter equipped with a resonant circuit, however, the phase potential changes relatively slowly during a commutation process, as exemplified in Fig. 8. The time which is relevant with respect to the generation of control signals of the modulator 30 is indicated in Fig. 8 by - the equivalent transition time is mentioned here. This time 1 represents the time when the phase voltage transition from one pole voltage to the other can be said to occur. In order for the converter to operate optimally, the converter should be controlled so that the commutation time rf specified by the modulator will correspond to the equivalent transition time z ,,. -u>: o 10 15 20 25 30 523 487 1.8 The equivalent transition time 1 ,, can be defined as follows: - When the transition of the phase voltage uphu), ie the change of the phase voltage uphu) in time, during a commutation process is "symmetrical" about the time axis the equivalent transition time 1 ,, coincides with the zero crossing of the phase voltage. Such a "symmetrical" case is illustrated in Fig. 8.

When the transition of the phase voltage uphu) during a commutation process is not "symmetrical" about the time axis, the equivalent transition time 1 ,, of the following conditions tlf + tc Iup ,, (z) dr = 0: lr -Ic where f, is so selected that the transition of the phase voltage up ,, (r) has not started at the time rw-z, and is completed at the time z ,, + r ,.

An example of such a "non-symmetrical" transition of the phase voltage up ,, (z) during a commutation process is illustrated in Fig. 9. The transition of the phase voltage up ,, (z) during a commutation process is "symmetrical" in those cases where uphno + f) = -u ,,,, (:, -f), for o where ro represents the time when the transition, ie the change, of the phase voltage uphu) begins and z, the time when the transition ends.

The equivalent transition time 1 ,, can be estimated on the basis of knowledge of the effect of the components included in the converter on the transition of the phase voltage uphu) during a commutation process. 10 15 20 25 30 35 | o u; According to the invention, in the execution of an intended commutation process ordered by the modulator 30, the control device 24 is caused to emit control signals to current valves 2, 3 and auxiliary valve 18 participating in the commutation process, for switching them on or off. , at times r, which are determined on the basis of a desired commutation time rf specified by the modulator 30, and a calculation algorithm, which is based on knowledge of the effect of the components included in the converter on the intended commutation process and whose input data consists of values of the phase current ip , and the intermediate voltage, wherein said calculation algorithm is designed taking into account the condition that the desired commutation time rf, will coincide with the equivalent transition time z, for the phase voltage.

Said calculation algorithm is suitably based on the value of the inductance LM of the resonant circuit 16, the value of the snubber capacitance C ,, and measured values of the intermediate voltage out, and out, respectively, and the phase current i ”. With the help of these parameters it is possible to determine with sufficient accuracy at which times z, the control device 24 shall emit control signals to the converter current valves and auxiliary valve so that the extinguishing semiconductor elements of the current valves and the extinguishing semiconductor components of the auxiliary valve are turned on and off. different commutation processes appropriate moments.

The current phase current ip, at the time of commutation, is registered with a suitable means for current measurement, schematically indicated at 31 in Fig. 6, which means is arranged to transmit measurement signals to the control device 24.

The converter suitably comprises means for measuring the voltage out, across the respective intermediate capacitor 7, 8, schematically indented at 32 in Fig. 6, which means is arranged to transmit measurement signals to the control device 24. In the event that a | »; ø »10 15 20 25 30 35 a. 5 '48 less accuracy in the control of the commutation processes can be accepted with a specific converter, it can be assumed that the voltage across the respective intermediate capacitor as ideally corresponds to half the pole voltage, said means 32 for voltage measurement being dispensable.

The values of the parameters used in the above-mentioned calculation algorithm, such as the snubber capacitance CS and the inductance Lm », are suitably determined by measurements performed on the inverter after it has been assembled. These measurements can advantageously be performed on repeated occasions during the life of the converter to compensate for changes in said values. This ensures that no adverse effect on the control of the commutation processes is caused by the gradual changes of the said values which arise due to the degenerations of the components of the resonant circuit which have arisen over time. Advantageously, the values of the snubber capacitance C, the inductance LM and other parameters used in the calculation of the times z can be determined by recursive identification, which means that the values of these parameters are continuously updated and adjusted based on measurements made during previous commutation process.

According to a preferred embodiment of the invention, the variation of the phase current iph during the respective commutation process is taken into account when calculating the times z, for outputting control signals from the control device 24. This results in an increased precision in the control. The varying value of the phase current during the commutation process is suitably estimated by means of a linear model of the load connected to the phase socket 10.

Below are examples of suitable algorithms for determining the times r, for outputting control signals from the control device 24 to the current valves 2, 3 and the auxiliary valve 18 of an ARCP converter in connection with the previously described types of commutation processes. In the case of a commutation of the phase current ip not assisted by the resonant circuit, from a quenchable semiconductor element of a first current valve 2, 3 to a rectifier means of a second current valve 3, 2 can the control device be arranged to emit: - a switch-off signal to the first current valve at a first time 1 ,, which is given by the formula where 1 ,, is the estimated time delay from the switch-off signal being emitted from the control device 24 to the switch-off of the hail conductor element of the the first current valve is executed, and - an ignition signal to the second current valve at or after a second time f "given by the formula where 1 ,, is the estimated time delay from the time the ignition signal is emitted from the control device 24 to the ignition of the shear conductor element of the the second current valve is executed.

In the above formulas for 1 ,, and r ,, T, the duration of the actual commutation process, is preferably estimated according to the previously stated formula.

For increased accuracy of the control, it may be appropriate to also take into account the lead of the first flow valve sheathing element and adjust the calculation of the duration T, and thus the calculations of the times 1 ,, and 1 ,, with regard to this lead. The effect of the lead on the commutation process is suitably determined by measurements made on the inverter after it has been assembled, which measurements can be repeated during the service life of the inverter to reach 1023 20 25 30 35 nao ø uo 523 487 22 o 4 »en u think of changes in the aftermath caused by degeneration of the components of the inverter.

In a commutation process involving a commutation of the phase current ip assisted by the resonant circuit, from a quenchable semiconductor element of a first current valve 2, 3 to a rectifier means of a second current valve 3, 2, the control device 24 may be arranged to emit: - an ignition signal to the auxiliary valve 18 at a first time f ", which is given by the formula where: ds is the estimated time delay from the time the ignition signal is emitted from the control device 24 until the ignition of the intended semiconductor component of the auxiliary valve 18 is effected, - a blanking signal to the first the current valve at or before a second time z ", which is given by the formula 0 1 tnz = t" 'ET !!' tai: where rd, is the estimated time delay from the switch-off signal being emitted from the control device 24 to the switch-off of the semiconductor element at the first auxiliary valve, an ignition signal to the second flow valve is executed at a third time i ", given by the formula. 1 tu: _ 't ”+ ïT / l' is taken where rd, is the estimated time delay from the time the ignition signal is emitted from the control device 24 until the ignition of the semiconductor element of the second current valve is effected, and .l-» n 10 15 20 25 30 35 n I ø. no n 523 487 | ø o I | o I ,, - - Q. - u \ n ø nu 2 ° 3 - an extinguishing signal to the auxiliary valve 18 at or after a fourth time z ,,,, given by the formula _. 1 tm _ tv +: Tu "[those where: die is the estimated time delay from the time the extinguishing signal is emitted from the control device 24 until the extinguishing of the intended semiconductor component of the auxiliary valve 18 is carried out.

In the event that 1, ”is greater than or equal to rd, the control device 24 can of course be arranged to deliver the extinguishing signal to the first current valve at the same time as the ignition signal to the auxiliary valve. In the same way, the control device 24 can be arranged to deliver the extinguishing signal to the auxiliary valve at the same time as the ignition signal to the second current valve in the event that: die is greater than or equal to 145. In the above formulas for z ,,,, rm, f ", and 1," is T ,, the duration of the current commutation process, preferably estimated according to the previously stated formula.

For increased accuracy of the control, it may be appropriate to also take into account the conduction of the semiconductor element of the first current valve and / or the power losses in the resonant circuit and / or irregularities in the voltage distribution between the intermediate capacitors and thus the calculation of the duration T the times z ,,,, rm, t ", and r ,, _, with regard to these factors. The influence of said factors on the commutation process is suitably determined by measurements made on the converter after it has been assembled, which measurements can be repeated under the converter. lifetime to compensate for changes caused by degenerations of the components of the inverter. ».» n | 10 15 20 25 30 35 c I uc now: S23 487 2'4 In a commutation process involving a commutation of the phase current iph assisted by the rectifier circuit from a rectifier means of a first flow valve 2, 3 to a quenchable semiconductor element of a second flow valve 3, 2 can be controlled the correction 24 be arranged to emit: - an ignition signal to the auxiliary valve 18 at a first time r ,,,, which is given by the formula -l fll 2 f !! where rd, is the estimated time delay from the time the ignition signal is emitted from the control device 24 until the ignition of the intended semiconductor component of the auxiliary valve 18 is effected, - an extinguishing signal to the first current valve at or before a second time z is given by the formula I 1 tm: = trf 'Tm': Tres' is taken where: ds is the estimated time delay from the extinguishing signal being emitted from the control device 24 until the extinguishing of the semiconductor element of the first auxiliary valve is effected, - an ignition signal to the the second flow valve at a third time r ,,,, given by the formula. 1 until _ t »+ 5 Tre: '[49 where zdg is the estimated time delay from the time the ignition signal is emitted from the control device 24 until the ignition of the semiconductor element of the second current valve is effected, and - an extinguishing signal to the auxiliary valve 18 at or after a fourth time point z ,,,,, given by the formula 10 15 20 25 30 35 nnu ø ro a 523 487 25. 1 inch = t ”+ 5 Three; + Tur 'inches where rm is the estimated time delay from the time the extinguishing signal is emitted from the control device 24 until the extinguishing of the intended semiconductor component of the auxiliary valve 18 is effected.

In the event that: da is greater than or equal to rd, the control device 24 can of course be arranged to output the extinguishing signal to the first current valve at the same time as the ignition signal to the auxiliary valve. In the same way, the control device 24 can be arranged to deliver the extinguishing signal to the auxiliary valve at the same time as the ignition signal to the second current valve in the case that rm is greater than or equal to r '.

In the above formulas for r ,,,,, rm, r ,,,, and r ,,, 4, TM is the duration of the resonant period, TW the duration of the ramp-up period and TH, the duration of the ramp-up period, are preferably estimated according to previous specified formulas.

For increased accuracy of the control, it may be appropriate to also take into account the conduction of the rectifier means of the current valves and / or the power losses in the resonant circuit and / or irregularities in the voltage distribution between the intermediate capacitors 7, 8 and thus adjust the calculations TW, T ", and TM of the times r ,,,,, rm, r ,,, 3 and r ,,,,, with regard to these factors.The effect of said factors on the commutation process is suitably determined by measurements performed on the converter after it has been mounted together. - which measurements can be repeated during the life of the inverter to compensate for changes caused by degeneration of the components of the inverter.

The above time delays rd, -r ,,, 0 can be functions of various variables, mainly the phase current. The time delays rd, - rm at different values of the phase current can for example be determined »1111 10 15 20 25 30 o o | | no 525 487 26 by measurements made on the inverter after it has been assembled. These measurements can advantageously be repeated at different times during the life of the converter to compensate for changes in said time delays caused, for example, by degenerations of the components of the converter.

The starting point for the above formulas for determining the times z, is that the commutation time rf, specified by the modulator 30, will correspond to the above-described equivalent transition time 1 ,, with the required accuracy.

It will be appreciated that the extinguishing and ignition of a switch valve semiconductor semiconductor element described above and specified in the claims refers to the simultaneous extinguishing and ignition of all extinguishable semiconductor components 13 of a current valve in cases where the respective current valve comprises a plurality of series-connected circuits 12 of previous described type. Likewise, it will be appreciated that the above and described extinction or ignition of an auxiliary valve extinguishable semiconductor component refers to the simultaneous extinguishing or ignition of an auxiliary valve all energized and energized extinguishable semiconductor components 20, respectively, in cases where the auxiliary valve 18 includes a plurality of of the type previously described.

The invention is of course not in any way limited to the preferred embodiments described above, but a number of possibilities for modifications thereof should be obvious to a person skilled in the art, without this departing from the basic idea of the invention as defined. in the appended patent claims.

Claims (11)

10 15 20 25 30 35 523: f _ ': j_I§ _ = -: "27 PATENTKRAV A method for controlling a VSC converter, which comprises - one between two poles (4, 5), a positive and a negative, at a voltage side of the converter arranged series connection of at least two current valves (2, 3), which current valves each comprise an extinguishable semiconductor element and a rectifier means connected thereto, an alternating voltage phase line (11) being connected to a center point (10), called a phase socket, of the series connection between two current valves while dividing the series connection into two equal parts, an intermediate conductor arranged on the voltage side of the converter comprising at least one intermediate conductor capacitor (7, 8), a resonant circuit (16) comprising at least one capacitive means (15), an inductor (17) and an auxiliary valve (18) provided with extinguishing semiconductor components for recharging said capacitive means (15) in connection with commutation of the phase current, - a control device (24) for controlling the switching on and off of the current valves extinguishing semiconductor elements and the extinguishing semiconductor components of the auxiliary valve, and - means (31) for measuring the phase current (iph), said ignition and extinguishing being controlled by the control device (24) depending on control desired indicating control signals received from a modulator (30) (2,1) for commutation, "characterized in that the control device (24), when performing an intended commutation process and ordered by the modulator (30), emits control signals to current valves (2, 3) and auxiliary valve participating in the commutation process. (18), for switching them on or off, at times (:,.) Determined on the basis of a desired commutation time (r, ',) specified by the modulator (30) and a calculation algorithm, which is based on knowledge of what effect the components included in the converter have on the intended commutation process, wherein said calculation algorithm is designed taking into account the condition that the desired commutation the time point (r, ',) shall coincide with an equivalent transition time (zu) for the phase voltage, which equivalent transition time (rn) is given by the condition Ilf + tC juphmdr = 0' Ir-te where up ,, (z) is the phase voltage, 1 ,, is the equivalent transition time and r, is so chosen that the change of the phase voltage (up ,, (z)) during the intended commutation process has not started at the time rW-zc and is completed at the time r, , + rc. . Method according to claim 1, characterized in that the value of the intermediate voltage, the value of the phase current (i ,,,,), the value of the snubber capacitance (Cs) and the value of the inductance (L) fe 'of the resonant circuit (16) are taken into account in said calculation algorithm. . Method according to Claim 2, characterized in that the value of the inductance (Lm) of the resonant circuit (16) and / or the value of the snubber capacitance (CS) are determined by measurements carried out on the converter. . Method according to claim 3, characterized in that the measurements for determining the value of the inductance (LM) of the resonant circuit (16) and / or the value of the snubber capacitance (CS) are performed repeatedly during the lifetime of the converter to compensate for changes in these values. . Method according to Claim 3, characterized in that the value of the inductance (LM) of the resonant circuit (16) and / or the value of the snubber capacitance (CS) are continuously updated and adjusted based on measurements made during previous commutation processes. 10 15 20 25 30 35 .u u ... .. .. _, ,,! ::: '. ::: - ::: --- "-'. ':;' ~ 1". -u- o - f-i SUG-I '.'-. . . '' "I '=; .. r._n n aíne a n g Method according to one of Claims 2 to 5, characterized in that the value of the voltages (μm) across the intermediate capacitors (7, 8) is determined by continuous measurements. Method according to any one of claims 2-6, characterized in that the variation of the phase current (iph) during the intended commutation process is taken into account in said calculation algorithm. Method according to Claim 7, characterized in that a linear model of the load connected to the phase socket is used to describe the variation of the phase current during the intended commutation process. Method according to one of the preceding claims, characterized in that parameter values included in the calculation algorithm are continuously updated and adjusted based on measurements made during previous commutation processes. Method according to one of the preceding claims, characterized in that the converter is constituted by an ARCP converter, the intermediate link comprising a series connection of at least two intermediate capacitors (7, 8) arranged between the two poles (4, 5) of the DC side of the converter. , and the resonant circuit (16) comprises a series connection of an inductor (17) and an auxiliary valve (18) arranged between the phase socket (10) and a center point (9) of said series connection of intermediate capacitors (7, 8), which auxiliary valve (18) comprises at least two extinguishing semiconductor components (20) arranged in opposite polarity relative to each other, the resonant circuit further comprising capacitive means (15) each connected in series with said inductor (17) and auxiliary valve (18). and in parallel with one of said flow valves (2, 3): Method according to claim 10, characterized in that in a commutation process involving a commutation of the phase current (iph) not assisted by the resonant circuit from a quenchable semiconductor element of a first current valve (2, 3) to a rectifier means of a second flow valve (3, 2), the control device is caused to emit: - an extinguishing signal to the first flow valve at a first time (rn) given by the formula tn = tfr'_T1'td1 2 where 1 ,, is said first time, rf, the desired commutation time given by the modulator (30), T, the estimated duration of the commutation process, and rd, the estimated time delay from the output of the quench signal (24) to the quenching of the semiconductor device; the element of the first flow valve is executed, and - an ignition signal to the second flow valve at or after a second time (rn) given by the formula. 1 t12_tu + ïT1'td2 where 1,2 is said second time and rd, the estimated time delay from the time the ignition signal is emitted from the control device (24) until the ignition of the semiconductor element of the second current valve is effected. Process according to Claim 11, characterized in that T 1 is given by the formula where ip 1 is the phase current, C, the snubber capacitance and U, the voltage across the intermediate link. A method according to claim 12, characterized in that the formula for calculating T, is adjusted with respect to the aftermath of the semiconductor element of the first current valve. Method according to one of Claims 10 to 13, characterized in that in a commutation process involving a commutation of the phase current (iph) assisted by the resonant circuit from an extinguishing semiconductor element of a first current valve (2, 3) to a rectifier means of a second current valve ( 3, 2), the control device (24) is caused to emit: - an ignition signal to the auxiliary valve (18) at a first time (rm) given by the formula where rm is said first time, rf, the desired commutation time given by the modulator (30 ), T ,, the estimated duration of the commutation process, and rd, the estimated time delay from the time the ignition signal is emitted from the control device (24) until the ignition of the intended semiconductor component of the auxiliary valve (18) is effected, - an extinguishing signal to the the first flow valve at or before a second time (rm) given by the formula 1 tm = t; 'ET !! "td4 where f", is said second time and rd, is the estimated time delay from the output of the quenching signal from the control device (24) to the execution of the quenching of the semiconductor element of the first auxiliary valve, an ignition signal to the second current valve at a third time (zm) given by the formula 10 15 20 25 30 523 487 32 where r ", is said third time and rd, is the estimated time delay from the time the ignition signal is emitted from the control device (24) until the ignition of the semiconductor element of the second current valve is executed, and - an extinguishing signal to the auxiliary valve (18) at or after a fourth time (z ”4) given by the formula. 1 + ET "" tdø where z "4 is said fourth time and: dö is the estimated time delay from the extinguishing signal being emitted from the control device (24) to the extinguishing of the intended semiconductor component of the auxiliary valve being carried out. , characterized in that T "is given by the formula 2Z -i T" = -l- zr-Zarctan -oI-ph-l wo Ud where i "is the phase current, U" the voltage across the intermediate, w = -1-- and Z = Lf ", where C is the snubber capacitance 0 L 0 C x sen and LM is the inductance of the resonant circuit (16). A method according to claim 15, characterized in that the formula for calculating T 'is adjusted with regard to the conduction of the semiconductor element of the first current valve and / or the power losses in the resonant circuit and / or irregularities in the voltage distribution between the intermediate capacitors (7, 8). Method according to one of Claims 10 to 16, characterized in that in a commutation process involving commutation of the phase current (flm) assisted by the resonant circuit from a rectifier means of a first current valve (2, 3). ) to an extinguishing semiconductor element of a second current valve (3, 2), the control device (24) is caused to emit: an ignition signal to the auxiliary valve (18) at a first time (r ,,,,) given by the formula where r, ,,, is said first time, rf, the desired commutation time given by the modulator (30), 22 ,: the estimated duration of the resonant period, TW the estimated duration of the ramp-up period and t), the estimated time delay from the that the ignition signal is emitted from the control device (24) until the ignition of the intended semiconductor component of the auxiliary valve is effected, - an extinguishing signal to the first current valve at or before a second time (rm) given by the formula where rm is said second t id point and: ds is the estimated time delay from the switch-off signal being emitted from the control device 24 until the switch-off of the semiconductor element of the first auxiliary valve is carried out, - an ignition signal to the second current valve at a third time point (z ,,,,) given by the formula Û _ 1 tm: _ fr, +: Tres' is taken 10 15 20 25 30 35 523 487 34 where rm, is said third time and 4.9 is the estimated time delay from the time the ignition signal is emitted from the control device ( 24) until the ignition of the semiconductor element of the second current valve is effected, and - an extinguishing signal to the auxiliary valve (18) at or after a fourth time (rm) given by the formula. 1 tur-z = t »+ ET + T 'tdio TEI I' where rm is said fourth time, T", the estimated duration of the ramp-down period and rm is the estimated time delay from the time the extinguishing signal is emitted from the control device (24) The method according to claim 17, characterized in that: T ", given by the formula where L", is the inductance of the resonant circuit (16), in the phase current, out, the voltage between a first of the poles (4, 5) and the center point (9) of the intermediate link when commutating the phase current from a current valve (2, 3) arranged between said first pole (4, 5) and the phase socket (10) to a current valve (3, 2) arranged between the second pole (5, 4) and the phase socket (10), while ud, is the voltage between the second pole (5, 4) and the center point (9) of the intermediate link when commutating the phase current from a current valve (3, 2) arranged between the second pole (5, 4) and the phase socket (10) of a flow valve (2, 3) arranged between the the first pole (4, 5) and the phase socket (10), 10 15 20 25 30 1523 487 35 TN, is given by the formula where u), the voltage between the second pole (5, 4) and the midpoint of the intermediate joint (9 ) when commutating the phase current from a current valve (2, 3) arranged between the first pole (4, 5) and the phase socket (10) to a current valve (3, 2) arranged between the second pole (5, 4) and the phase socket (10 ), while udj. is the voltage between the first pole (4, 5) and the center point (9) of the intermediate link when commutating the phase current from a current valve (3, 2) arranged between the second pole (5, 4) and the phase socket (10) to a current valve (2, 3) arranged between the first pole (4, 5) and the phase socket (10), and TW is given by the formula where wo = Method according to claim 18, characterized in that the formula for calculating Tm is adjusted with regard to the aftermath of the rectifier means of the current valves and / or the power losses in the resonant circuit and / or irregularities in the voltage distribution between the intermediate capacitors (7, 8). Method according to one of Claims 11 to 19, characterized in that the respective time delay (zu-rdw) is estimated on the basis of a measured value of the phase current "(ip ,,). A method according to any one of the preceding claims, characterized in that the control device (24 ) receives the control signals indicating the desired commutation times (1,1) from a PWM modulator.